1. Field of the Invention
This invention relates to the field of fiber optics, and more particularly to an active optical device, such as a transmitter and/or receiver device for coupling to an external light guide, such as an optical fiber cable.
2. Description of Related Art
In the field of optical fiber transmission, optical devices are needed to generate light for transmission into, and receive light from, the optical fibers. The actual transmitting devices are typically VCSELS (Vertical Cavity Surface Emitting Lasers), which are highly efficient semiconductor light emitting devices. These optical devices need to be associated with driver circuits that supply the appropriate current to modulate the lasers. The receivers are typically PIN diodes, which also need to be associated with appropriate driver circuitry to demodulate the received light signals. Tie drivers must be electrically connected to external circuits.
VCSELS typically are arranged in banks of four on a single chip. In the prior art, it is known to mount them on a xe2x80x9clead framexe2x80x9d, which consists of a metal frame, usually made of nickel, having separate leads that connect the individual VCSELs to their respective driver circuits. Prior art devices operating at 2.5 Gbps typically have their VCSELs mounted on the end face of a solid rectangular metal block that acts as a heat sink. A printed circuit containing the driver chips is mounted on one face of the heat sink, and the lead frame bonded to the heat sink connects the driver chips to the VCSEL contacts. The end face of the heat sink has protruding guide pins that precisely locate a fiber optic ferrule relative to the VCSELs so that the optic fibers contained in the optic fiber cable are respectively aligned with the individual optical components, whether they transmitters or receivers. The driver chips should be mounted as close to the VCSELs as possible to minimize parasitic effects. In the prior art, this is achieved by mounting them at the end of the metal block forming the heat sink.
While the above arrangement works well at 2.5 Gbs, there is an ever increasing desire to increase speed, and a new generation of devices will operate at 10 Gbs. These new devices create a need for a further improvement in parasitic performance, and in particular require the drivers to be mounted even closer to the VCSEL light sources.
The invention departs from the traditional method described above and mounts all the components on a substrate, typically made of glass or sapphire, using a solder alignment technique to mount the components in precise and close proximity. Unlike the prior art, light transmission takes place through the transparent substrate with the components being mounted on th)e rear face and the light guide being attached to the front face by means of guide pins protruding on either side of the transparent substrate.
Accordingly in a first aspect the invention provides a method of making an active optical device for coupling to an external light guide, comprising the steps of providing a substrate providing a light path therethrough and having a front face and a rear face; providing a plurality of components for attachment to die rear face of said substrate, each said active component having a face presenting an array of contacts, said components including at least one optical component selected from the group consisting of a light emitter and light receiver; forming a plurality of arrays of contacts on the rear face of said transparent substrate at precisely defined locations corresponding to an intended location of the contacts of each component; flip-chip bonding said components onto said substrate using a solder alignment technique to attach said components to said substrate in precisely predetermined locations determined by said arrays of contacts; and said at least one optical component being oriented so that it can be optically coupled through said transparent substrate to the external light guide.
Ideally, the substrate is transparent to the operating wavelength, although if necessary holes can be forced in the substrate to create a light path therethrough.
The active optical components can be VCSEls (Vertical Cavity Surface Emitting Lasers) serving as light emitters and PIN diodes serving as light receivers. The device is designed for use at 10 Gbs at a wavelength of 850 nm, although it is also useful at other speeds and wavelengths, including 240 Gbps.
It will be understood by one skilled in the art that fiber optic transmission can occur in the visible and non-visible portions of tile spectrum, and the terms xe2x80x9copticalxe2x80x9d and xe2x80x9clightxe2x80x9d are used herein to include those portions of the spectrum, including infrared and ultraviolet, normally used for fiber optic transmission whether visible or not.
The substrate can be any suitable material, but it is ideally transparent to the operating wavelength to avoid the need to form holes, and glass or sapphire are preferred. Sapphire is the most preferred due to its beat transmission properties.
Typically, the substrate is first patterned to provide the conductive tracks in a conventional manner, and then solder nodules known as xe2x80x9cbumpsxe2x80x9d are applied by deposition through a mask to permit the components to be attached by flip-chip bonding. Alternatively, the bumps can be applied to the corresponding contacts on the components, or both. Flip-chip bonding is a technique, for example, described in U.S. Pat. No. 6,190,940, incorporated herein by reference, wherein chips are bonded xe2x80x9cupside downxe2x80x9d directly onto a substrate. Matching contacts are present on the substrate and chip, and solder bumps are deposited on the substrate, the chip or both. The parts are brought together and the solder heated. As the solder melts, it draws the parts into precise alignment by surface tension effects and also bonds them together. The solder bumps, as is known in the art, are typically made of a gold/tin or gold/tin/lead alloy.
By mounting the components on a substrate using flip-chip bonding techniques and having the light exit from the other side of the substrate, it is possible to mount them to very fine tolerances and also to bring the driver circuits into close proximity with the active optical components.
A remaining problem is how to align the ferrule containing the optic fibers providing the external light guide with the VCSELs. This is achieved by using the same solder alignment technique to mount a guide frame, typically made of plated nickel, on the rear side of the substrate. Solder bonding pads are laid out in a predetermined pattern, and the guide frame has matching pattern which is solder aligned with the bonding pads. The guide frame has wing portions overhanging the side of the substrate, and the wing portions contain indicia, preferably in the form of through-holes, marking the location of the guide pins. Thus, in a final step, when a heat sink, typically a block of highly conductive metal, is applied to the rear side of the substrate, the guide pins, which protrude from the heat sink and pass on each side of the substrate, can be aligned relative to the rest of the components, and more particularly the optical components, by passing them through the guide holes in the guide frame.
In another aspect the invention provides an active optical device comprising a substrate with a light path therethrough and having a front face and a rear face; a plurality of components solder bonded to the rear face, said components including at least one active optical component located so that it can be coupled through the transparent substrate to in external light guide; a guide frame solder bonded to the rear face of the substrate; and a heat sink having guide pins bonded on the rear side of said substrate, said guide pins protruding forward to engage recesses in an external light guide coupling.